1. Field of the Invention
The present invention is directed to ink jet printing systems, and in particular to drop-on-demand ink jet printing systems having printheads with heater elements.
2. Description of the Related Art
Ink jet printing systems can be divided into two types. The first type is a continuous stream ink jet printing system and the second type is a drop-on-demand printing system.
In a continuous stream ink jet printing system, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed so that the stream breaks up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital date signals end passed through electrostatic field which adjusts the trajectory of each droplet in order to direct the ink droplets to e gutter for recirculation or to a specific location on a recording medium.
In a drop-on-demand ink jet printing system, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless the droplet is to be placed on the recording medium. Because the drop-on-demand ink jet printing item requires no ink recovery, charging or deflection, such system is much simpler than the continuous stream ink jet printing system. Thus, ink jet printing systems are generally drop-on-demand ink jet printing systems.
Further, there are two types of drop-on-demand ink jet printing systems. The first type uses a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle. The second type uses thermal energy to produce a vapor bubble in an ink-filled channel to expel an ink droplet.
The first type of drop-on demand ink jet printing system has a printhead with ink-filled channels, nozzles at ends of the channels and piezoelectric transducers near the other ends to produce pressure pulses. The relatively large size of the transducers prevents close spacing of the nozzles, and physical limitations of the transducers result in low ink drop velocity. Low ink drop velocity seriously diminishes the tolerances for drop velocity variation end directionality and impacts the system's ability to produce high quality copies. Further, the drop-on-demand printing system using piezoelectric transducers suffers from slow printing speeds.
Due to the above disadvantages of printheads using piezoelectric transducers, drop-on-demand ink jet printing systems having printheads which use thermal energy to produce vapor bubbles in ink-filled channels to expel ink droplets are generally used. A thermal energy generator or heater element, usually a resistor, is located at a predetermined distance from a nozzle of each one of the channels. The resistors are individually addressed with an electrical pulse to generate heat which is transferred from the resistor to the ink.
The transferred heat causes the ink to be super heated, i.e., far above the ink's normal boiling point. For example, a water based ink reaches a critical temperature of 280.degree. C. for bubble nucleation. The nucleated bubble or water vapor thermally isolates the ink from the heater element to prevent further transfer of heat from the resistor to the ink. Further, the nucleating bubble expands until all of the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor which, of course, removes heat due to heat of vaporization. During the expansion of the vapor bubble, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus.
When the excess heat is removed from the ink, the vapor bubble collapses on the resistor, because the heat generating current is no longer applied to the resistor. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as an ink droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity to expel the ink droplet towards a recording medium, such as paper, in a substantially straight line direction. The entire bubble expansion and collapse cycle takes about 20 microseconds (.mu.s). The channel can be retired after 100 to 500 .mu.s minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to be somewhat dampened.
To eject an ink droplet, each heater element must become hot enough to cause the ink to reach a bubble nucleation temperature of preferably 280.degree. C. for water based ink. In order for the heater element to generate the thermal energy to cause bubble nucleation, an operating voltage is applied to a resistor of the heater element. Typically, the operating voltage is proportional to the resistance of the resistor, i.e., the higher the resistance, the higher the operating voltage.
Conventionally, polysilicon is used to form the resistors of the heater elements. The resistance value of the resistors is chosen based on the actual required power (Power=V.times..vertline.=.vertline..sup.2 .times.R=V.sup.2 /R) for ejection of the ink droplet through bubble nucleation. Once the required power and voltage has been chosen, the resistance value is determined. The fabrication of the determined resistance is controlled by the sheet resistance (ohm/square; .OMEGA./.quadrature.) of the polysilicon and the size of the resistor. The size of the resistor can be tightly controlled by photolithographic techniques. The sheet resistance of the polysilicon is primarily controlled by impurity doping, preferably by ion implantation, and annealing of the ion doped polysilicon.
FIG. 1 illustrates the variation of sheet resistance of a wafer of p-type polysilicon doped with conventional ion implantation and annealing process. The lines in FIG. 1 represent contour lines and each contour line represents an increase (+) or a decrease (-) of the sheet resistance by 1% from the mean sheet resistance. Thus, a large number of contour lines indicates greater deviation from the mean sheet resistance. As shown, the sheet resistance within a length of the wafer varies by 12.80% and typically, the sheet resistance can vary from 10% to 15%. Thus, a plurality of resistors formed by ion implantation during the fabrication of the heater elements results in variation in sheet resistance between the resistors. Because the size of the plurality of the resistors are the same and the sheet resistance varies by 10% to 15%, the resistance of the resistors between each other will vary by 10% to 15%.
Although highly resistive polysilicon loads with sheet resistance in the order of 2 to 4 K.OMEGA./.quadrature. are used in static RAM design, the sheet resistances of resistors used in thermal ink jet application must be both highly accurate, e.g., about 40 .OMEGA./.quadrature., and tightly controlled. Variations in resistance between the resistors have adverse effects on the operation of the heater elements and the lifetime of the heater elements, which in turn, will affect the operation and lifetime of the printhead. When the chosen voltage is applied to a resistor having a resistance greater than the desired resistance, a power less than the power required for bubble nucleation is generated, and thus, the ejection of an ink droplet is prevented. When the chosen voltage is applied to a resistor having a resistance less than the desired resistance, a power greater than the power required for bubble nucleation is generated, and such generated power causes ink to bake on the resistor to form an insulator layer between the ink and the resistor. The formation of the insulator layer on resistors of lower resistance and non-ejection of ink droplets by resistors of higher resistance require increase in the voltage necessary to produce ink droplets over the lifetime of the printhead. Such increase in voltage shortens the operating lifetime of the printhead.
The following patents disclose various printheads having resistors made of polysilicon, but none of the patents discloses substantially uniform sheet resistance between the resistors of the heater elements and method of fabricating such resistors.
U.S. Pat. No. 4,947,193 to Desphande discloses an improved thermal ink jet printhead having a plurality of heating elements in ink channels selectively addressable by electrical signals to eject ink droplets from nozzles located at one end of the ink channels on demand. The heating elements each have a passivated layer of resistive material that has non-uniform sheet resistance in a direction transverse to the direction of ink in the channels. The non-uniform sheet resistance provides a substantially uniform temperature across the width of the resistive layer, so that the power required to eject a droplet is reduced and the droplet size dependence on electrical signal energy is eliminated.
U.S. Pat. No. 4,370,660 to Hara et al. discloses a liquid ejecting recording process using a liquid ejecting recording head comprising a liquid discharging portion including an orifice for ejecting liquid droplets and a heat acting portion communicated with the orifice. The heat acting portion is a portion where heat energy for discharging liquid droplets acts on a liquid, and an electrothermal transducer has a structure laminated on a substrate with the layers in the following order: a lower layer, a resistive heater layer, and an upper layer from the substrate to the heat acting portion on the position of the heat acting portion. When a signal voltage is applied to the resistive heater layer and potentials V.sub.A and V.sub.B are applied to two electrodes A and B, a potential V applied to at least the surface portion of the upper layer is kept intermediate between V.sub.A and V.sub.B while the signal voltage is applied to the resistive heater layer.
In "Wafer Charging Control in High-Current Implanters" by Wu et al., Wu et al. examines and reviews wafer charging in high-current ion implanters, and the operation of the electron flood gun in the Varian 160-10 implanter. It is shown that flood gun electrons with energies up to 350 eV do reach the wafers and can cause damage when wafers are excessively overflooded. An in situ flood gun monitor using a capacitive pickup sensor is described. Experiments with the capacitive charge sensor have further shown that (i) wafers can self-charge during pump-down or venting of the target chamber, (ii) a slight overflooding is preferable to underflooding, and (iii) for perfect neutralization, the flood gun emission current should vary with the magnetic scanning of the ion beam across the wafers. Using metal oxide semiconductor capacitors as test vehicles, Wu et al. shows that other factors can also affect charging damage to devices during implantation, such as the thickness of the field oxide or photoresist relative to the ion penetration depth, the proper grounding of the back sides of wafers during implantation, and the polarity of the silicon underneath the gate oxide. The benefits of proper electron flood control are demonstrated, and operating procedures are recommended.
U.S. Pat. No. 4,532,530 to Hawkins discloses a carriage type bubble ink jet printing system having improved bubble generating resistors that operate more efficiently and consume lower power without sacrificing operating lifetime. The resistor material is heavily doped poly-crystalline silicon which can be formed on the same process lines with those for integrated circuits to reduce equipment costs and achieve higher yields. Glass mesas thermally isolate the active portion of the resistor from the silicon supporting substrate and from the electrode connecting points so that the electrode connection points are maintained relatively cool during operation. A thermally grown dielectric layer permits a thinner electrical isolation layer between the resistor and its protective ink interfacing tantalum layer and thus increases the thermal energy transfer to the ink.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.